A separator for electrochemical batteries refers to an intermediate membrane that isolates a cathode and an anode from each other in a battery while maintaining ionic conductivity, thereby enabling charge/discharge of the battery.
Recently, along with a trend of pursuing light weight and miniaturization of electrochemical batteries to improve portability of electronic equipment, there is a need for high-power, high-capacity batteries for electric vehicles. Thus, a separator for batteries is required to have reduced thickness and weight as well as excellent dimensional stability under heat and high tension so as to improve productivity of high-capacity batteries.
In addition, recently, not only general properties such as permeability and thermal shrinkage resistance, but also properties influencing lifespan of batteries such as cycle characteristics, or properties influencing productivity of batteries such as electrolyte injection capabilities are also taken into account as important factors in manufacture of separators.
Among various processes in manufacture of batteries, since a winding process requires a substantial period of time and is closely associated with productivity of batteries, winding is performed at a high rate in order to increase production speed. Thus, in order to prevent process failure due to breakage of separators upon winding at a high rate, there is a need for studies on increase in tensile strength of separators.
As one example of techniques for manufacture of separators in the related art, Japanese Patent Publication No. 2004-083866 discloses a method for manufacturing a polyolefin porous membrane through simultaneous biaxial stretching of an ultrahigh molecular weight polyethylene film twice at different temperatures. However, this method is focused on improvement of air permeability and thermal shrinkage through adjustment of fine pores of the film, and is far from a technique capable of achieving increase in tensile strength of separators, particularly, improvement in productivity of separators.